Longwinded post ahoy!
I want to start this discussion here, as it’s a topic that crops it’s head up regularly. I have noticed a large portion of posters generally respond with the same advice, ventilate, open windows, don’t dry cloths etc. But that this advice is given so often and the opinion is so strong, that it is done to the exclusion of other causes and solutions (particularly on this forum but also elsewhere).
I feel, and some may find this controversial, adding extra ventilation by opening windows or vents is both correct in most circumstances, but also a deeply flawed sticking plaster solution and only correct due to poor building standards, and it does not deal with the underlying cause, and can cause uncomfortably low temperatures.
Firstly let’s look at the underlying principle of condensation occurring, and that is warmer more humid air, hitting a colder surface (cold air holds less moisture), and let’s look at a number of examples, recommended humidity levels in homes from a number of sources recommend 30-50%, I believe 40c to be reasonably average, my office for example right now is currently 50%.
At 18C, and 40% humidity, a surface needs to be 4c for condensation to occur.
At 21C, and 40% humidity, a surface needs to be 7c for condensation to occur.
At 18C, and 50% humidity, a surface needs to be 7c for condensation to occur.
At 21C, and 50% humidity, a surface needs to be 10c for condensation to occur.
As I type this, the outside air humidity is 80%, I personally consider a 30% humidity in homes to be unrealistic, but would hear views on this.
The basic point I wish to address is that people should consider the surface temperatures of areas where condensation is occurring, before yelling “open your windows”.
If the outside air temperature is near 0-4c, is it unreasonable to assume some surfaces will reach the dew points listed above?
Consider that windows are often of poor quality, or set within brickwork surrounds with no thermal breaks and steel lintels, that wall insulation does often not lap up with roof insulation, leaving corners un-insulated, consider many buildings don’t even have insulation!
All the areas that show up as problems, opening windows does not solve these cold surface areas, it simply lowers the indoor air temperature relative to the wall or window surfaces, so solving condensation but not by solving the underlying cause.
Now, I do understand this is a DIY forum, and so telling people to upgrade insulation or to fix cold bridges rather than open a window may be more “appropriate”. But I saw a thread recently where someone had missing thermal breaks in double glazed windows, that resulted in -2c temperatures inside at the bottom of the frame on cold nights (confirmed later by thermal imaging cameras, it was up norrrth). Even after this was determined, several people still insisted that the problem was a lack of ventilation.
To put this into context, you would need.....
at 35% humidity to stop condensation occurring on a surface at -2c, an indoor temperature no higher than 13C, sod that!
I want to start this discussion here, as it’s a topic that crops it’s head up regularly. I have noticed a large portion of posters generally respond with the same advice, ventilate, open windows, don’t dry cloths etc. But that this advice is given so often and the opinion is so strong, that it is done to the exclusion of other causes and solutions (particularly on this forum but also elsewhere).
I feel, and some may find this controversial, adding extra ventilation by opening windows or vents is both correct in most circumstances, but also a deeply flawed sticking plaster solution and only correct due to poor building standards, and it does not deal with the underlying cause, and can cause uncomfortably low temperatures.
Firstly let’s look at the underlying principle of condensation occurring, and that is warmer more humid air, hitting a colder surface (cold air holds less moisture), and let’s look at a number of examples, recommended humidity levels in homes from a number of sources recommend 30-50%, I believe 40c to be reasonably average, my office for example right now is currently 50%.
At 18C, and 40% humidity, a surface needs to be 4c for condensation to occur.
At 21C, and 40% humidity, a surface needs to be 7c for condensation to occur.
At 18C, and 50% humidity, a surface needs to be 7c for condensation to occur.
At 21C, and 50% humidity, a surface needs to be 10c for condensation to occur.
As I type this, the outside air humidity is 80%, I personally consider a 30% humidity in homes to be unrealistic, but would hear views on this.
The basic point I wish to address is that people should consider the surface temperatures of areas where condensation is occurring, before yelling “open your windows”.
If the outside air temperature is near 0-4c, is it unreasonable to assume some surfaces will reach the dew points listed above?
Consider that windows are often of poor quality, or set within brickwork surrounds with no thermal breaks and steel lintels, that wall insulation does often not lap up with roof insulation, leaving corners un-insulated, consider many buildings don’t even have insulation!
All the areas that show up as problems, opening windows does not solve these cold surface areas, it simply lowers the indoor air temperature relative to the wall or window surfaces, so solving condensation but not by solving the underlying cause.
Now, I do understand this is a DIY forum, and so telling people to upgrade insulation or to fix cold bridges rather than open a window may be more “appropriate”. But I saw a thread recently where someone had missing thermal breaks in double glazed windows, that resulted in -2c temperatures inside at the bottom of the frame on cold nights (confirmed later by thermal imaging cameras, it was up norrrth). Even after this was determined, several people still insisted that the problem was a lack of ventilation.
To put this into context, you would need.....
at 35% humidity to stop condensation occurring on a surface at -2c, an indoor temperature no higher than 13C, sod that!
